TBC (Truck Body Controller) fuses should have been labeled MUCH better by the inbred designers and manual writers. But the BCM (what most schematics call it) manages the two dimmable lamp feeds, so that's the connection. There's one for incandescent lamps and another one for dash panel LEDs. LEDs and incandescent lamps respond slightly differently to the PWM signal used for dimming (LEDs are more linear), so the designers included two in the vehicle. To be more accurate, changing lights from incandescent to LED should also involve changing the feed bus from one to the other, but I've never heard of anybody being that anal.